In U.S. Pat. No. 4,063,176, assigned to the assignee of the present invention, there is disclosed a high frequency broadband mixer having a balun arrangement with each of two signal ports having a pair of ground plane strip conductor baluns and a single dielectric substrate, together with a frequency converter connected between said baluns. In using this type of balun it was necessary to terminate the strip conductors for each balun at the frequency converter and use cross-over connections between the strip conductors in different planes to provide signal polarities that will convert from unbalanced to balanced signals and at the same time isolate the two signal ports that receive RF and LO signals from one another.